This invention relates to novel semi-permeable gas separation membranes based upon uncross-linked polymers containing meta, para-bisphenol moieties and a process for separating gases using said membranes.
The use of semi-permeable membranes to separate gases is well known in the art. Membranes have been used to recover or isolate a variety of gases including hydrogen, helium, oxygen, nitrogen, carbon monoxide, carbon dioxide, water vapor, hydrogen sulfide, ammonia, and/or light hydrocarbons. Applications which are of particular interest include the separation of hydrogen or helium from gas mixtures such as mixtures containing nitrogen, carbon monoxide, carbon dioxide, water vapor, and/or light hydrocarbons. For example, the separation and recovery of hydrogen for recycle is often necessary in various hydrocracker, hydrotreater, and catalytic cracking processes used in the oil refinery industry. Other applications of interest include the separation of carbon dioxide from light hydrocarbons or other crude oil components such as hydrogen sulfide as part of the tertiary oil recovery process. Additional applications include the recovery of an enriched oxygen stream from air for use in fermentation processes or to enhance combustion in furnaces. Alternatively, an enriched nitrogen stream may be obtained from air for use as an inert atmosphere over flammable fluids or to enhance the storage longevity of perishable foodstuffs. Membranes can be used to accomplish such separations.
Such membrane separations are based on the relative permeability of two or more gaseous components through the membrane. In order to separate a gas mixture into two portions, one of which is richer and one of which is leaner in at least one gaseous component, the feed mixture is brought into contact with one side of a semi-permeable membrane through which at least one of the gaseous components selectively permeates. A gaseous component which selectively permeates through the membrane passes through the membrane more rapidly than at least one other gaseous component of the mixture. The gas mixture is thus separated into a stream which is enriched in the selectively permeating gaseous component or components and a stream which is depleted in the selectively permeating gaseous component or components. A relatively non-permeating gaseous component passes more slowly through the membrane than at least one other gaseous component of the mixture. An appropriate membrane material is chosen so that some degree of separation of the gas mixture can be achieved.
Membranes for gas separation have been fabricated from a wide variety of natural and synthetic polymeric materials, including rubbers, polysiloxanes, cellulose esters and ethers, aromatic polyimides, polyaramides, polysulfones, polyethersulfones, polyesters, and polycarbonates. An ideal gas separation membrane is characterized by the ability to operate under high temperatures and/or pressures while possessing a high gas separation factor (permselectivity) and high gas permeability. While solvent resistance under operating conditions is also preferred, gas separation membranes are preferably fabricated from polymers which are easily processed. The problem is finding membrane materials which possess all the desired characteristics. Polymers which possess high separation factors generally have low gas permeabilities, while those polymers which possess high gas permeabilities generally have low permselectivities. In the past, a choice between a high gas separation factor and a high gas permeability has been unavoidably necessary. Furthermore, some polymeric membrane materials which would otherwise be considered useful exhibit poor performance under high operating temperatures and pressures. Still other polymeric membrane materials capable of operating at high temperatures and pressures are typically difficult to fabricate into membranes. Solvent resistance is often obtainable only with polymeric materials which are difficult to fabricate into membranes. What is needed is a membrane capable of separating gas mixtures and which possesses a high permselectivity, high gas permeability, flexibility to operate under extreme conditions of temperature and pressure, as well as improved solvent resistance under operating conditions and ease of fabrication.